Field of the Present Disclosure
The present disclosure relates to a negative electrode for a lithium secondary battery, and more particularly, to a negative electrode for a lithium secondary battery having high negative electrode efficiency and excellent capacity retention, and a lithium secondary battery including the negative electrode.
Discussion of the Related Art
Conventionally, lithium metal is used as a negative-electrode active material of a lithium battery. When lithium metal is used, there is a risk of explosion resulting from short-circuiting of the battery due to the formation of dendrite. Thus, instead of lithium metal, a carbon-based material is used as a negative-electrode active material.
The carbon-based active material includes crystalline carbon, such as graphite and artificial graphite, and amorphous carbon, such as soft carbon and hard carbon. However, although the amorphous carbon has a large capacity, irreversibility is large in the charging and discharging process. As the crystalline carbon, graphite is typically used, and the theoretical limit capacity thereof is 372 mAh/g. Thus, graphite has a high capacity and is used as a negative-electrode active material.
However, even though the theoretical capacity of such graphite or carbon-based active material is relatively high, its theoretical capacity is only 380 mAh/g. Therefore, such graphite or carbon-based active material may not be used as a negative electrode active material in the development of a high capacity lithium battery in the future.
In order to overcome such problems, currently-studied active materials are metal-based or intermetallic compound-based negative-electrode active materials. For example, researches have been conducted in which metals or semimetals such as aluminum, germanium, silicon, tin, zinc, lead and the like are used as negative-electrode active materials. These materials have high capacity and high energy density and may absorb and release more lithium ions than the negative-electrode active materials using carbon-based materials. Thus, they may be used to make batteries with high capacity and high energy density. For example, pure silicon is known to have a high theoretical capacity of 4017 mAh/g.
However, the metal-based or intermetallic compound-based negative-electrode active materials have lower cycle characteristics, compared with carbon-based materials. Thus they are not yet practical. This is due to the following reasons: When using pure silicon itself as a negative-electrode active material which is a lithium absorption and emissive material, conductivity between the active materials may deteriorate due to the volume change during charging and discharging, and the electrode active material may be peeled from the negative current collector. That is, the silicon contained in the negative-electrode active material absorbs lithium during charging and thus expands to about 300 to 400% of its original volume. When lithium is released, the inorganic particles thereof shrink.
Repeating such charge and discharge cycles may cause electrical insulation due to cracks in the negative-electrode active material, resulting in a drastic reduction in battery life.
Therefore, in order to solve such a problem, Korean Patent Application No. 10-2014-0165114 filed by the present applicant discloses a metallic negative active material having a significantly improved expansion ratio. In addition, Korean Patent Application No. 10-2015-0001837 filed by the present applicant discloses that amorphization of such metal-based negative active material is preferably in the range of 25% or more.
Thus, a negative electrode having improved performance may be manufactured by blending the metal-based material, especially the silicon-based negative active material, with the conventional graphite-based negative active material. In this connection, there is an increasing need to fabricate the negative electrode for secondary batteries using an alloy/graphite blend-based negative-electrode active material via the proper blending of the metal-based, in particular, silicon-based negative active materials and the graphite-based negative active materials.